Audio system

ABSTRACT

An audio system comprises a headset comprising a pair of earpieces positionable with respect to the user&#39;s ears so that, in use, the user is inhibited from hearing sounds from the user&#39;s environment, at least one of the earpieces having a transducer for reproducing sounds from an input audio signal; a microphone for generating a captured audio signal representing captured sounds from the user&#39;s environment; a detector for detecting the presence of any of a set of one or more characteristic sounds in the captured audio signal; and a controller operable to transmit sounds from the user&#39;s environment to the user in response to a detection that one or more of the set of characteristic sounds is present in the captured audio signal.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of the earlier filing date ofGB1207903.4 filed in the United Kingdom Intellectual Property Office on4 May 2012, the entire content of which application is incorporatedherein by reference.

BACKGROUND

1. Field

This invention relates to audio systems.

2. Description of Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, is neitherexpressly or impliedly admitted as prior art against the presentdisclosure.

A head-mountable display (HMD) is an image or video display device whichmay be worn on the head or as part of a helmet. Either one eye or botheyes are provided with small electronic display devices.

Some HMDs allow a displayed image to be superimposed on a real-worldview. This type of HMD can be referred to as an optical see-through HMDand generally requires the display devices to be positioned somewhereother than directly in front of the users eyes.

Some way of deflecting the displayed image so that the user may see itis then required. This might be through the use of a partiallyreflective mirror placed in front of the user's eyes so as to allow theuser to see through the mirror but also to see a reflection of theoutput of the display devices. In another arrangement, disclosed inEP-A-1 731 943 and US-A-2010/0157433, a waveguide arrangement employingtotal internal reflection is used to convey a displayed image from adisplay device disposed to the side of the user's head so that the usermay see the displayed image but still see a view of the real worldthrough the waveguide. Once again, in either of these types ofarrangement, a virtual image of the display is created (using knowntechniques) so that the user sees the virtual image at an appropriatesize and distance to allow relaxed viewing. For example, even though thephysical display device may be tiny (for example, 10 mm×10 mm) and maybe just a few millimetres from the user's eye, the virtual image may bearranged so as to be perceived by the user at a distance of (forexample) 20 m from the user, having a perceived size of 5 m×5 m.

Other HMDs, however, allow the user only to see the displayed images,which is to say that they obscure the real world environment surroundingthe user. This type of HMD can position the actual display devices infront of the user's eyes, in association with appropriate lenses whichplace a virtual displayed image at a suitable distance for the user tofocus in a relaxed manner—for example, at a similar virtual distance andperceived size as the optical see-through HMD described above. This typeof device might be used for viewing movies or similar recorded content,or for viewing so-called virtual reality content representing a virtualspace surrounding the user. It is of course however possible to displaya real-world view on this type of HMD, for example by using aforward-facing camera to generate images for display on the displaydevices.

Although the original development of HMDs was perhaps driven by themilitary and professional applications of these devices, HMDs arebecoming more popular for use by casual users in, for example, computergame or domestic computing applications.

In head mountable displays where the user sees only the displayedimages, and potentially hears only the generated sounds provided by aheadset, the user is shielded from interaction with the real world. Sucharrangements can be referred to as “total immersion” meaning thatsubstantially all of the user's attention is taken up by the audio andvideo supplied by the head mountable display. While this can give animproved audio/video or gaming experience, it can lead to health andsafety problems because the user is unaware of outside events.

SUMMARY

This invention provides an audio system comprising:

a headset comprising a pair of earpieces positionable with respect tothe user's ears so that, in use, the user is inhibited from hearingsounds from the user's environment, at least one of the earpieces havinga transducer for reproducing sounds from an input audio signal;

a microphone for generating a captured audio signal representingcaptured sounds from the user's environment;

a detector for detecting the presence of any of a set of one or morecharacteristic sounds in the captured audio signal; and

a controller operable to transmit sounds from the user's environment tothe user in response to a detection that one or more of the set ofcharacteristic sounds is present in the captured audio signal.

Further respective aspects and features of the invention are defined bythe appended claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the present technology.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 schematic illustrates a noise cancelling headset;

FIG. 2 schematically illustrates an earpiece for use in a headsetsimilar to that shown in FIG. 1;

FIGS. 3-5 schematically illustrate possible detector-controllerarrangements;

FIG. 6 schematically illustrates a head mountable display (HMD);

FIG. 7 schematically illustrates the generation of a virtual image;

FIG. 8 schematically illustrates a movable shutter;

FIG. 9 schematically illustrates an electrically alterable shutter;

FIG. 10 schematically illustrates the control of video reproduction inan HMD in dependence upon object detection;

FIG. 11 schematically illustrates a camera-based object detectionarrangement;

FIG. 12 schematically illustrates an emitter-receiver based objectdetection arrangement;

FIG. 13 schematically illustrates an accelerometer-based detectionarrangement;

FIG. 14 schematically illustrates a passive infrared detection system;

FIG. 15 is a schematic flowchart illustrating the operation of theheadset of FIG. 1 or the HMD of FIG. 6 in respect of the reproduction ofaudio signals;

FIG. 16 is a schematic flowchart illustrating the operation of the HMDof FIG. 6 in respect of the reproduction of video signals;

FIG. 17 schematically illustrates a data processing system comprising anHMD and a camera arrangement;

FIG. 18 schematically illustrates an image captured by the cameraarrangement of FIG. 17;

FIG. 19 schematically illustrates a stereoscopic camera and microphonearray;

FIG. 20 schematically illustrates the operation of a microphone array;

FIG. 21 schematically illustrates a plan view of a room;

FIG. 22 is a schematic flow chart of operations involved in detectinganother object near a wearer of an HMD; and

FIGS. 23 to 25 are schematic flowcharts illustrating operations carriedout in response to a detection.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various techniques will be described in connection with embodiments ofthe present invention. Some of these techniques relate to the handlingof audio signals. Such techniques are applicable to embodiments whichdeal only with audio reproduction (such as audio headsets) or toembodiments which deal with audio and video reproduction (such as headmountable displays with audio reproduction facilities). Others of thetechniques relate to the handling of video signals. Such techniques areapplicable to embodiments which deal only with video reproduction (suchas head mountable displays) or to embodiments which deal with audio andvideo reproduction (such as head mountable displays with audioreproduction facilities).

Referring now to the drawings, FIG. 1 schematic illustrates a noisecancelling headset 10 comprising a pair of earpieces 20, 30, one foreach of a user's ears when in use, together with a support band 40 whichpasses over the user's head to support the earpieces 20, 30.Accordingly, the earpieces are positionable with respect to the usersears so that, in use, the user is inhibited from hearing sounds from theuser's environment. At least one of the earpieces (preferably both) hasa transducer for reproducing sounds from an input audio signal. It willbe appreciated that the support band 40 may pass behind (rather thanover) the users head, or in other embodiments the support band 40 maynot in fact be needed, if the earpieces 20, 30 are of such a size andshape that they may be supported merely by friction with the innersurface of the ear, or (for example) by individual over-the-ear clips.

Each earpiece 20, 30 includes a respective audio transducer 50connectable to an audio signal source for reproducing as an audibleoutput electrical signals which are supplied to the transducers via acable or wireless connection 60. The earpieces may each comprise soundinsulating formations to inhibit the transmission of sound from theenvironment to the user's ears.

Embodiments of the invention relate to a noise cancelling headset, andthe arrangement shown in FIG. 1 includes features relevance to noisecancellation. It will be appreciated however that the noise cancellationaspects are not essential to the present embodiments, and thedifferences applicable to a non-noise cancellation headset will bediscussed below.

A microphone 70 detects ambient noises around the headset 10, and inparticular generates a captured audio signal representing capturedsounds from the users environment. The aim of a noise cancellationsystem is to pass a signal to the transducers 50 which depends upon theambient noise but is in anti-phase with the noise. So, it is appropriatethat the microphone 70 should be disposed near to the earpieces 20, 30.In one example shown in FIG. 2, a respective microphone 70 is mounted toeach of the earpieces 20, 30 (only one earpiece being shown in FIG. 2for clarity). In other embodiments, a microphone may be provided atanother part of the system, for example as part of a control unitdisposed part-way along a cable providing the connections 60 to thetransducers 50.

To provide noise cancellation, captured signals from the microphone (ormicrophones) 70 are passed to an amplifier 80 and an inverter 90 so asto provide an amplified and anti-phase version of the ambient noise. Theamplifier and inverter therefore represent a noise reducing processoroperable to generate an anti-phase version of an audio signalrepresenting environmental noise, and to transmit the anti-phase versionto the earpiece transducers for replay to the user. In a simple noisecancellation arrangement, the amplified and anti-phase signal is simplymixed with audio signals which are passed to the transducers 50 forreproduction. There are a few differences in respect of the presentembodiments, and these will be described below. Note that in FIG. 1, themixing function, in normal operation, is carried out by a controller 110which will be described below. The controller 110 receives signals fromthe inverter 90 and also an input audio signal for reproduction by theheadset. A user control (not shown) may be provided to allow the user toadjust one or both of: the gain of the amplifier 80; and the mixingproportions between the input audio signal and the output of theinverter 90.

The skilled person will appreciate that the term “noise cancellation”does not necessarily imply a complete cancellation or removal of ambientnoise. Indeed, a 100% noise removal would be unlikely to be achieved.Instead, the term implies a noise reduction by at least a partialcancellation of at least some components of the ambient noise.

The amplifier 80 and inverter 90 need not necessarily be implemented asseparate units; an amplifier could be used which, by virtue of standardfeatures of its implementation, provides a phase-inverted output. Itwill also be appreciated that in arrangement such as that shown in FIG.2, the noise cancellation system 85 comprising the amplifier 80 and theinverted 90 may be physically provided within the respective earpiece20, 30.

In the present embodiments, noise cancellation is at least partiallysuspended in respect of certain categories of ambient noise.

A potential issue with a number of different types of so-calledimmersive devices such as noise-reduced headsets and head mountabledisplays is that they have the potential to occupy the whole of theuser's attention. While this can be a desirable feature if the user isengaged in listing to a piece of music, or playing a video game, orwatching a film, or trying to sleep on an aeroplane, it could mean thatthe user is prevented or inhibited from hearing sounds relating topotential safety hazards or other matters requiring attention in thevicinity of the user. Examples of the types of sounds which the userprobably should hear but which may be inhibited by a noise reducingheadset include alarm sounders such as fire alarm sounders, doorbells,other people wishing to gain the user's attention, crying babies and soon. In the present embodiments, various techniques are used to detectsuch sounds and allow the sounds to be brought to the users attention.In general terms, the characteristic sounds represent sounds of at leasta threshold volume and/or having a characteristic frequency content, butthese and other options are discussed below.

One way of achieving this is shown in FIG. 1, the headset is providedwith a detector 100 and a controller 110. The detector 100 receivesaudio signals either from the microphone 70 or, if one is provided, fromthe amplifier 80. The detector 100 detects the presence, in the audiosignal received by the detector 100, of particular categories of sound,or in other words the presence of any of a set of one or morecharacteristic sounds in the captured audio signal. Examples ofdetection techniques will be discussed below. In response to thedetection of a sound within one of the particular categories, thecontroller 110 can carry out one or more of the following actions:

a) temporarily prevent or reduce the effect of noise cancellation, forexample by inhibiting the mixing of the signal from the inverter 90 intosignals to be passed to the transducers 50;

b) temporarily extinguish or reduce the level of the audio signal forreproduction which is passed to the transducers 50; and/or

c) temporarily pass in-phase signals from the microphone 70, suitablyamplified, to the transducers 50.

The controller is therefore operable to transmit the captured audiosignal, at least in part, to the one or more transducers (or to allowthe user to hear the users environment in a direct sense) in response toa detection that one or more of the set of characteristic sounds ispresent in the captured audio signal.

Accordingly, the controller acts as a mixer having three inputs: theanti-phase output from the inverter 90; the input audio signal forreproduction; and the in-phase amplified output from the microphone 70,provided by the detector 100. In normal operation, which is to say, whena particular sound has not been detected by the detector 100, the mixingfunction of the control 110 provides a mix between the input audiosignal and the output of the inverter 90. As mentioned above, a usercontrol may be provided to establish the relative levels of these twosignals in normal operation, so as to give a noise cancellation or noisereduction function which is appropriate for that user.

When the detector 100 detects a particular sound which is to be broughtto the users attention, the controller 110 can take one or more of theactions (a)-(c) listed above.

Action (a) is implemented by altering the mixing proportions applied bythe controller 110 so as to reduce the proportion or level of the mixrepresenting the output of the inverter 90. In turn, this reduces thecancellation of ambient noise which can allow the user to detectexternal noise is more easily, of course including the noise which is tobe brought to the users attention.

Action (b) is implemented by altering the mixing proportions applied bythe controller 110 so as to reduce the level of the input audio signalwhich is supplied to the transducers 50.

Action (c) is implemented by altering the mixing proportions applied bythe controller 110 so as to increase (potentially from a normal value ofzero) the level of the amplified in-phase signal derived from themicrophone 70 in the signals supplied to the transducers 50.

Examples of the mixing levels applied by the controller 110 in thevarious different circumstances are shown in the following table, whichuses linear rather than logarithmic indications of level, such that alevel of “0” indicates that none of a signal is passed to thetransducers 50 as part of the output mix of the controller 110, and alevel of “1” indicates that the whole of the signal is passed to thetransducers 50 as part of the output mix of the controller 110. Notealso that as each of the actions (a)-(c) can be implementedindependently, only those changes relevant to a particular action areindicated. Note also that the values shown below are purely for examplepurposes; other values could be used, and either a substantiallyinstantaneous transition to the new value could be used, or a gradualtransition over (say) 100 milliseconds could be applied, or thetransition could be selected in dependence upon the type of noise (suchas a fast transition for a fire alarm, or a slower transition for a babycrying).

Main audio signal Anti-phase In-phase level microphone level microphonelevel Normal Operation 1 0.3 0 Action (a) → 0 Action (b) →0.3 Action (c)→ 1

Any one or more of these actions, taken by the controller 110, will tendto emphasise the external noise in the category selected by the detector100 in preference to the reproduction of the main audio signal.

It will be appreciated that an operation which is equivalent toincreasing the mixer gain for the in-phase microphone level can beachieved simply by increasing the magnitude of the in-phase microphonesignal which is passed to the mixer.

Other arrangements are possible. For example, the earpieces 20, 30 couldcomprise mechanical vents which are normally closed so as to inhibitsounds from reaching the user's ears, but which can open under thecontrol of the controller 110 so as to allow external sounds to reachthe user's ears. The vents could be, for example, spring-loaded so as tobe held in a closed position by a latch, electromagnet or other holdingarrangement against the influence of a spring or other resilient mountwhich would tend to open the vents. The controller 110 can cause therelease of the holding arrangement so that the vents are released tomove, under the influence of the spring or resilient mounting, to theiropen position. The user could manually return the vents to theirlatched-closed position after the detected sound has been attended to.

The arrangements are not limited to noise cancelling headsets of thetype just described. Other headsets do not make use of active noisecancellation, but instead just inhibit ambient noise from reaching theuser's ears by providing sound insulation as part of the earpieces 20,30. The sound insulation may comprise a generally ring-shaped formationof flexible insulating material which provides a seal, in use, aroundthe user's ears. In such examples, at least the inverter 90 is notprovided, and the function of the amplifier 80 may be incorporatedwithin the functionality of the detector 100.

In response to a detection of a sound in one of the categories detectedby the detector 100, either or both of actions (b) and (c) listed aboveis relevant, and (purely by way of example) may lead to the followingadjustments in mixing carried out by the controller:

Main audio signal In-phase level microphone level Normal Operation 1 0Action (b) →0.3 Action (c) → 1

The arrangement described above relating to movable mechanical vents isalso relevant to this type of headset.

Note that some headsets may combine both types of arrangement, in thatthey may use active noise cancellation (using an amplifier 80 and aninverter 90) and may also make use of passive noise insulationtechniques such as those described above.

Note also that the techniques described here can apply to devices suchas head mountable displays, telephone hands-free headsets, helmets withbuilt-in headsets, aviation headsets, electronic stethoscopes and thelike, which make use of audio reproduction in such a way as to inhibit(passively, actively or both) ambient noise.

FIGS. 3-5 schematically illustrate possible detector and/or controllerarrangements.

The arrangements of FIGS. 3-5 provide examples of some ways of detectingcategories of external noise to be brought to the attention of thewearer of the headset using one or more of the techniques describedabove.

In FIG. 3, signals from the microphone 70 are passed to a digitalprocessor 120 acting as the detector. The digital processor users knownspeech recognition techniques to detect the presence of speech withinthe signals captured by the microphone 70. If speech is detected, amixer 130 (acting as part of the controller 110) is either controlled bythe digital processor 122 increase its level for the signals from themicrophone 70, or is supplied with an increased level microphone signalby the digital processor 120.

FIG. 4 uses a so-called “squelch” circuit or processor 140. This is anon-linear processor which acts under the control of a level detector150 which in turn is responsive to the level of the signal from themicrophone 70. When the signal level is below a threshold, the squelchcircuit 140 does not pass the microphone signal to the mixer 130. Whenthe signal level as detected by the level detector 150 is above thethreshold, the squelch circuit 140 passes the microphone signal to themixer 130. The combination of the squelch operation and the mixingoperation mean that ambient noise of more than a threshold volume ispassed to the mixer to be mixed into the signals supplied to thetransducers 50, whereas ambient noise below the threshold volume is notpassed to the mixer.

FIG. 5 illustrates a filter 160 such as a band-pass filter, the outputof which is supplied to a comparator 170 which compares the filteroutput with a threshold value. The comparator 170 controls the operationof the mixer 130 according to any one or more of the actions (a)-(c)discussed above, so as to preferentially pass the microphone signalrepresenting the in-phase external noise when the filtered microphonesignal is detected by the comparator 170 to exceed a threshold signallevel.

FIG. 6 schematically illustrates a head mountable display (HMD).

The HMD of FIG. 6 comprises a frame 200 including portions 210 which cansecure the frame to the ears of the user, earpieces 220, displayportions 230 and masking portions 242 excluded some or all ambient lightfrom the user's eyes while the user is operating the HMD. The displayportions 230 and the earpieces 220 are supplied with video and audiosignals for reproduction by a games machine or data processor 250.Accordingly, the HMD of FIG. 6 represents an example of an HMD having aframe to be mounted onto an observers head, the frame defining one ortwo eye display positions which, in use, are positioned for viewing bythe observer. In embodiments of the invention the frame also comprisesone or more shutters for normally obscuring the observer's view of thesurrounding real environment. A display element may be mounted withrespect to each of the eye display positions, the display elementproviding a virtual image of a video display of a video signal from avideo signal source to the observer.

The earpieces 220 may form part of a system corresponding to thatdescribed with reference to FIGS. 1-5. In this regard, the microphone ormicrophones 70 may be mounted anywhere on or with respect to the frame200 and/or the earpieces 220 so as to capture ambient sounds in thevicinity of the user while the user is wearing the HMD. The functions ofthe detector 100 and the controller 110 may be provided by electronicsmounted on or with respect to the frame 200 or by functionality of thegames machine or data processor 250. The games machine or data processor250 may communicate with the HMD by a wired or wireless connection.

Of course, the HMD of FIG. 6 does not necessarily need to provide thenoise-handling functionality described with reference to FIGS. 1-5.

The HMD shown in FIG. 6 provides images for display to the users eyes insuch a way that the user can see only the displayed images (or at least,the displayed images form the predominant part of the user's view). Themasking portions 240 help to achieve this by reducing the amount ofambient light incident upon the users eyes. The display portions 230themselves are opaque in that they do not pass ambient light to theuser’ eyes. So, the act of viewing images on the display portions 230 isanother example of a substantially immersive experience for the user, inthat the user is at least inhibited from seeing other things which wouldotherwise be visible in the vicinity of the user.

The display portions 230 are physically very close to the user's eyes.However, the user is actually able to view a virtual image in respect ofeach display portion 230, the virtual image being significantly largerthan the actual physical size of the display portion 230 (or a displayelement forming an active part of the display portion 230) and beingsignificantly further away than the distance between the eye and thedisplay element of the display portion 230.

FIG. 7 schematically illustrates the generation of a virtual image. Theuser's eye 260 views an image presented on a display element 270 througha lens arrangement 280. The lens arrangement 280 is shown forschematically this is as a single convex lens, but of course could be acompound or more complex arrangement of lenses and optical elements.Using known techniques, the effect of the lens arrangement 280 is toprovide the user with a view of a virtual image 290 on a virtual imageplane 300 separated from the users eye by a distance of, for example, 3m. In general terms, embodiments of the invention the virtual image isgenerated at a distance of more than 1 m from the HMD frame.

In a similar way to the audio embodiments discussed above, and aim ofthe present embodiments is to detect visual events of a type categorisedsuch that the user should be informed of the occurrence of those events,and then to provide a technique for allowing the user to be notified ofsuch an occurrence.

As an example, if a moving object approaches to within a thresholddistance of the user's head (and/or approaches at at least a thresholdrate), it is appropriate that the user should be made aware of themoving object to avoid any possibility of injury. Here, it is noted thatthe immersive nature of the viewing of image or video material using theHMD of FIG. 6 means that without techniques such as the ones to bedescribed below, the user may not notice the moving object approachingthe users head. Other types of occurrence which may be notified to theuser, and techniques for notifying the use of such occurrences, will bediscussed below.

Accordingly, embodiments of the invention provide an object detector fordetecting the presence of an object within a detection range of theobserver's head, for example by detecting an object approaching thehead, and a controller for controlling operation of the HMD so as toallow the observer to view at least a portion of the surrounding realenvironment in response to a detection of the presence of an objectwithin the detection range, which may represent the detection of anobject approaching the head.

FIG. 8 schematically illustrates a movable shutter used in someembodiments of the present invention. The movable shutter 310 forms afront (outer) face of the display portion 230 and provides at least someof the light-obscuring functions of the display portion 230. An actuator320 controls movement of the movable shutter 310 between aview-obscuring position 330 and a non-obscuring (or less-obscuring)position 340 so that in the non-obscuring or less obscuring position theuser may view at least a portion of the surrounding real environment.There are various ways of achieving this movement. In some embodiments,the actuator 320 may be a motor drive or an electromagnetic driveoperable to move the movable shutter 310 in either direction, that is,from the position 330 to the position 340 or from the position 340 tothe position 330. However, noting that this arrangement may useexcessive electrical power in the context of what is possibly abattery-powered HMD, and also noting that the arrangement may provide atime lag between a detection being made that the user's view should beunobscured and the actual unobscuring taking place, in another type ofembodiment the actuator 320 comprises a spring or other resilientelement operable to move the movable shutter 310 from the obscuringposition 330 to the non-obscuring position 340, and a latching mechanismsuch as a mechanical or electromagnetic latch, releasable underelectrical control, to hold the movable shutter 310 against theoperation of the resilient element in the position 330.

FIG. 9 schematically illustrates an electrically alterable shutter asanother type of arrangement. Here, the display portion 230 comprises aregion of a material 350 having an electrically alterable opticaltransmission, which can be varied between a more opaque state (a viewobscuring mode) and a less opaque state (a view allowing state) inresponse to an electrical signal 360 generated by the controller.

In a further possible arrangement, a camera 370 such as a forward-facingcamera may be provided as part of the HMD. In order to selectivelyprovide the user with a view of his environment in response to thedetection of a hazard or other occurrence, instead of changing theopacity of the HMD, and image of the surrounding environment can beselectively displayed by the display portions 230 based on imagescaptured by the camera 370.

Techniques for generating an electrical signal to control these types ofarrangements will now be described.

FIG. 10 schematically illustrates the control of video reproduction inan HMD in dependence upon object detection. In FIG. 10 a generic objectdetector 400 is illustrated. Possible configurations of the objectdetector 400 will be discussed below. A video source 410 represents thesource of image or video material to be displayed by the displayportions 230. For example, the video source 410 may be embodied by thegames machine or data processor 250. A shutter or video controller 420provides the control to change the nature of the display portions 230between an obscuring and a non-obscuring configuration (in the case ofvariable shuttering as described with reference to FIGS. 8 and 9) or tochange the type of video material which is displayed by the displayportions in the case of systems using an HMD-mounted camera such as thecamera 370.

In particular, if the object detector 400 detects an object within acertain distance and/or positional relationship with respect to the HMD,then if a shuttering system is used, the shutter or video controller 420controls the shuttering system to change to a less obscuring or lessopaque condition so that the user can see (at least in part) the user'sexternal environment. If a camera such as the camera 370 is used, thenin response to detection of an object as described above, the shutter orvideo controller 420 can control the display portions 232 display videoreceived from the camera 370 in preference to video received from thevideo source 410. Note that the transition between the two video sourcesdoes not need to be absolute; the video from the camera 370 can be mixedwith the video from the video source 410. The mixing may be on a specialbasis, so that the video from the camera 370 is inserted into a portionof the displayed image, similar to a picture-in-picture system. All themixing can be such that the video from the camera 370 is superimposedover the video from the video source 410.

FIG. 11 schematically illustrates a camera-based object detectionarrangement, for example making use of the camera 370 forming part ofthe HMD of FIG. 6. Accordingly, embodiments of the invention provide acamera mounted sellers to move with the frame of the HMD, and an imagecomparator operable to compare successive images captured by the cameraso as to detect the presence, in a captured image, of an object that wasnot at that image position in a previously captured image.

Images from the camera 370 are passed to an image comparator 430 and animage buffer 440. The image buffer 440 stores the captured images sothat the image comparator 430 may compare a current captured image withone or more previously captured images so as to detect imagedifferences. The image differences may be compared with a threshold.This can be done in various different ways. For example, a sum ofabsolute pixel value differences may be detected with respect to thewhole image or one or more sub-portions of the image, as a comparisonwith a previous image. The sum of absolute pixel value differences givesa single variable indicative of image difference, which can be comparedwith the threshold. As an alternative, in order to detect an objectapproaching the user within view of the camera 370, image differencescan be detected in such a way as to detect a region of the image whichgrows over the course of several images (for example, five images). Thesize of the detected region in the fifth image divided by the size ofthe detected region in the first image (of the group of five images upto and including a current image) provides a single variable indicativeof the approach of an object, which again can be compared with thethreshold. In either case, exceeding the threshold is indicative of adetection of an approaching object, with the detection being passed tothe shutter or video controller of FIG. 10.

Note that the images captured by the camera may be translated, beforethe comparison takes place for object detection, so as to avoidincorrect object detections caused simply by movement of the HMD betweencaptured images. The translation can be in response to detected motionof the HMD so as to aim to cancel that motion out from the imagecomparison. The motion may in turn be detected by one or more motiondetectors separate to the camera (such as an accelerometer at the HMD)or may be detected by detecting image motion of fixed points (or atleast points which have been present and stationary in multiple capturedimages) such as parts of the image background.

FIG. 12 schematically illustrates an emitter-receiver based objectdetection arrangement. Here, an emitter 450 emits a detection signalwhich is received after reflection from an approaching object by areceiver 460. The process can be controlled by a detector 470 whichcontrols the timing of emissions by the emitter 450 and detects thelevel of signals received by the receiver 460. The emitted and receivedsignals may be, for example, ultrasonic signals, radio-frequencysignals, infrared signals or the like.

FIG. 13 schematically illustrates an accelerometer-based detectionarrangement. Here, a detection is made of sudden motion of the user, andin particular sudden motion of the users head (assuming the user iswearing the HMD on his head), represented for example by detection of apredetermined characteristic motion. This is not the same as detectingan approaching object, but can indicate a situation in which the usershould be allowed to see the external environment to avoid a possiblehazardous situation. This type of detection can be used in place of theobject detection carried out by the object detector 400 of FIG. 10, orcan be used in addition to the object detection, for example bycombining the detection output of the arrangement of FIG. 13 with thedetection output of the object detector 400 by a logical OR operation.

Referring to FIG. 13, an accelerometer 480 is mounted with respect tothe HMD of FIG. 6 so that movements of the HMD are detected by theaccelerometer 480. A comparator 490 compares the detected accelerationswith a threshold value. If the magnitude of the detected acceleration isgreater than the threshold value, a detection is made of a potentiallyhazardous situation and the shutter or video controller 420 iscontrolled to allow the user to see the external environment. In avirtual reality arrangement, the video signal source can be operable toprovide a video signal for display having an apparent viewpoint which isdependent upon the detected motion.

In other embodiments, motion detection may be carried out by using acamera mounted so as to move with the frame of the head mountabledisplay; and an image comparator operable to compare successive imagescaptured by the camera so as to detect inter-image motion.

FIG. 14 schematically illustrates a passive infrared detection system asanother example of an object detector 400. A known passive infrared(PIR) head end 500 is associated with a motion detector 510 and mountedwith respect to the HMD so as to detect an approaching object.

It will be appreciated that the detection techniques described above arenot mutually exclusive, and that they can be combined in anycombination.

FIG. 15 is a schematic flowchart illustrating the operation of theheadset of FIG. 1 or the HMD of FIG. 6 in respect of the reproduction ofaudio signals.

At a step 600, characteristic sounds which should be made available tothe user are detected in the ambient environment external to the user.In instances where one or more of those sounds are detected in acaptured audio signal, at a step 610 the detected sounds arepreferentially reproduced in the earpiece or earpieces so that the usermay hear them.

FIG. 16 is a schematic flowchart illustrating the operation of the HMDof FIG. 6 in respect of the reproduction of video signals.

At a step 620, a detection is made of an approaching object or of motionof the user's head or both. At a step 630, the object size or speed, orthe head motion, compared with a threshold. If the motion or the objectsize or speed are greater than the threshold then the user is allowed tosee the external environment at a step 640. Otherwise, as a step 650, nochange is made to the user's view.

It will be appreciated that in an immersive environment such as thatdescribed above, very sudden changes could be disconcerting to the userand could even exacerbate a potentially hazardous situation. Steps canbe taken to alleviate this potential problem. For example, if the useris viewing a video game involving moving action, such as a car racinggame, then the object detector 400 could instruct the games machine 250to cause the users in-game motion to be brought to a halt before theexternal environment is allowed to be seen by the user. As anotherexample, changes can be made gradually, for example over a period of 100ms, to allow the user to become accustomed to the transition from theimmersive virtual environment to the view of the real external(surrounding) environment.

Other techniques for notifying the user are also possible. For exampleinstead of reproducing video of the external environment on the displayportions 230, warning messages such as “take the HMD off now” could bedisplayed by the display portions 230. In other alternatives, instead ofchanging the user's view, a warning message could be provided throughthe earpieces 220 as an audible warning, such as “sit down now”. Furtheroptions will be discussed below in connection with FIGS. 23-25.

FIG. 17 schematically listless the data-processing system comprising anHMD 700 worn by a user 710, and a camera arrangement 720 comprising adepth camera and microphone array 730 and a processor unit 740.

The depth camera and microphone array 730 captures images of theenvironment in which the user 710 is positioned. In embodiments of theinvention, one or more optical markers may be mounted with respect tothe HMD 700 in order to allow the processor unit 740 to distinguish theposition of the HMD 700 in the captured images. For example, such anoptical marker might be a passive markers such as a coloured shape orcould be an active markers such as a light emitting diode (LED) whichcould be continuously or intermittently illuminated. Intermittentillumination could be used in order to encode an identity of the HMDand/or of the user by means of a pulse coded illumination signal. Inother arrangements, the colour of the optical marker could indicate anidentity of the HMD and/or of the user. In some examples, the LED may bean infrared LED, generating light which is invisible to the human eye.One or more optical markers may be used. Depending upon the particularapplication of the HMD, it may be a normal expectation that the userwearing the HMD faces in the general direction of the camera 730, but inother applications and order to allow for the user turning his head toface a different direction, multiple optical markers may be spacedaround the periphery of the HMD so that at least one optical marker maybe observed whatever the current orientation of the users head whilewearing the HMD.

The processor 740 may be implemented as, for example, a Sony®PlayStation 3® games console running appropriate firmware and/orsoftware.

The depth camera and microphone array 730 and the processor unit 740cooperate to process images captured by the depth camera and to identifythe position in 3-D space of the user wearing the HMD. As mentionedabove, this may be by analysis of the images to detect the position ofoptical markers in the captured images.

FIG. 18 schematically illustrates an example image as captured by thecamera arrangement 720 of FIG. 17. This example image shows a portion ofa room with walls 750, for 760 and ceiling 770, while and the user 710wearing the HMD 700 within the room.

In example arrangements, the images captured by the camera arrangement720 may be used to provide an indication of a possible need to allow theuser to experience either or both of the ambient or surrounding audio orvisual environment. In this regard, the images captured by the cameraarrangement 720 may supplement or replace either or both of theproximity detection and audio detection arrangements discussed above.Techniques by which this can be performed will be discussed below.

Similar functions may be carried out using audio signals captured by thecamera arrangement 720. Again, techniques by which this may be performedwill be discussed below.

FIG. 19 schematically illustrates a stereoscopic camera and microphonearray, providing an example of the depth camera and microphone array 730of FIG. 17.

The arrangement shown in FIG. 19 comprises a casing 780 holding twolaterally spaced cameras 790 and five (in this example) laterally spacedmicrophones 800.

The cameras 790 cooperate together to capture stereoscopic images. Depthinformation can be derived using known techniques from such stereoscopicimages by analysis of parallax between the images captured at the spacedapart positions. Note that other types of depth camera are available andare suitable for use in respect of the present techniques. For example,a depth camera may include a light source which projects an array ofso-called structured light (in one basic example, a pattern such as agrid) onto the scene to be captured. The depth, or at least relativedepth, of items in the scene may be detected by detecting, in thecaptured image, distortions of the light pattern. The light projected bythe camera may be, for example, infra-red illumination and thereforeinvisible to the user. A separate camera element may be used to capturethe light pattern, which then allows the main image to be captured usinga camera element which can be made (using filters or the like)insensitive to the wavelength(s) used to project the patterned light. Inanother example arrangement, an ultrasonic or radio frequency radar-likedetector can be associated with a camera element, so as to capture depthinformation to be associated with the captured image data. Note alsothat if depth information is not used, and only lateral separation isused in the object detection processes described below, a 2D camera (notdetecting depth) can be used instead of a depth camera.

Accordingly, it will be understood that techniques other than the use ofstereoscopic cameras are also applicable to embodiments of the presentinvention.

It will also be appreciated that although the camera and microphonearray 730 are described in this example as a single unit, embodiments ofthe invention can operate with just a camera (if audio detection is notused) or with just a microphone or microphone array (if image baseddetection is not used).

The microphones 800 can operate as a phased array of microphones. Thisoperation will now be explained briefly in connection with FIG. 20which, just for clarity of the diagram, shows two example microphones.

Referring to FIG. 20, a sound source at a particular position 810 isdetected by spaced apart microphones at respective positions 820, 830. Adifference in audio phase will be detected between the differentmicrophones because of the different lengths of the audio paths 840, 850from the sound source at the position 810 to each of the microphones. Ifmore microphones are used, a different respective audio path will applyin respect of each of the microphones. This allows various differentmodes of operation. In one example, an estimate of the position of thesound source relative to the microphone array may be generated bydetecting the various phase differences for the same sound as receivedby different ones of the spaced apart microphones in the array. Inanother example, the microphone array may be used to preferentiallydetect sounds from a source at or near a certain spatial position byapplying a respective phase delay to the signals received by each of themicrophones in the array and summing the results, the phase delayscancelling out the phase differences which would apply to soundsarriving at each microphone from a source at the required position. Thislatter arrangement will tend to give destructive interference in respectof a sound source not at the required position and constructiveinterference in respect of a sound source at the required position. So,the microphone array may be used for position detection or for at leastpartially isolating sounds generated by sources at particular spatialpositions.

In other embodiments, it will be appreciated that a single microphoneelement could be used. A single element would not generally allow fordirectional or spatial information relating to the source of a sound tobe detected, but would allow for the capture of ambient sounds in thevicinity of the user wearing the HMD.

In other examples, instead of a phased array of microphones, an array ofdirectional microphones could be used, such that each one is pointedtowards a different respective direction. The direction, relative to themicrophone array, of a sound source can then be detected by detectingwhich of the microphones provides the greatest magnitude audio signalrelating to a particular sound. A sound source in a particular directioncan be selected by selecting the signal from a required one of themicrophones. So, it will be understood that alternatives to a phasedarray of microphones are envisaged within the scope of embodiments ofthe present invention.

As mentioned above, the stereoscopic images captured by the cameras 790may be analysed using known techniques to derive depth information inrespect of the environment as viewed by the cameras. FIG. 21schematically illustrates a plan view of a room containing the user 710.In order to compare the room of FIG. 21 with the room of FIG. 18, itshould be assumed that the depth camera is positioned at the bottom edgeof the plan view of FIG. 21, looking in towards the room. The depthinformation associated with the captured images, for example informationdefining a distance 860 between the depth camera and the user, can beencoded and handled as, for example, a depth value associated with eachpixel of the captured image of FIG. 18.

FIG. 22 is a schematic flowchart of operations involved in detectinganother object or person near a wearer of an HMD. The operations of FIG.22 relate first to the differentiation of fixed parts of the scene asviewed by the camera (the background such as walls, furniture and thelike) and variable parts of the scene such as the user 710 and anyapproaching objects or people.

The background of the image is detected in one of two example ways. In afirst example, the user may instruct the camera to capture an image ofthe room or other background without the user present (for examplebecause the user is positioned behind the camera at the time ofcapturing this image). This would form a background image for use inlater comparisons with foreground objects. In a second example, thesystem may detect those parts of the image which are present andunchanged at plural sample image points spread out in time. So, even ifthe user moves around, uncovering and covering areas of the background,those background errors will remain the same before and after theircovering and uncovering.

Referring to FIG. 22, in respect of a particular 3-D image as captured,as a step 900 the processor 740 subtracts the background image from thecurrent captured image. This leaves a residual image representingchangeable items such as the image of the user 710. At a step 910, theprocessor 740 detects objects in the captured image including the HMD700 and any nearby objects. Here, the HMD 700 may be identified by theoptical markers discussed above, and an object may be identified as anearby object if (i) there is a region of background image between thatobject and the HMD 700, and (ii) the nearby object is not itself part ofthe background, and (iii) the nearby object is within a thresholddistance to the HMD 700. The test relating to criterion (iii) is carriedout at a step 920 in FIG. 22.

The detection of whether the object is within a threshold distance(which may be considered as a detection range) of the user's head may becarried out in various ways. For example, in an arrangement in whichdepth information is available in association with a captured image, thelateral displacement of an object relative to the user's head may bedetected by detecting the lateral separation in the captured imagebetween the detected object and the detected HMD position. Here, lateralmay refer to horizontal, vertical or radial distance as detected in theplane of the image. The depth separation may be detected by detecting,from the depth data, the absolute difference between the detected depthof the HMD and the detected depth of the object.

The comparison with a threshold distance may be carried out by combiningthe lateral and depth displacements to obtain a single radial distancebetween the object and the HMD. For example, if the detected depth ofthe HMD is Dz (in metres), relative depth displacement is Lz (measuredin metres, for example) and the relative horizontal and verticaldisplacements are Nx and Ny (measured in pixels), the field of view ofthe camera is FOV degrees and the pixel resolution of the captured imageis Px·Py pixels (horizontal×vertical), then the following analysis maybe considered as an example of the present techniques:

-   -   (a) derive a horizontal lateral displacement, Lx, in metres:        angular horizontal displacement is approximately (Nx·FOV)/Px        lateral displacement in metres is approximately Dz·sin(angular        displacement) for small displacements, this approximates to        Dz·(angular displacement) so an approximation of Lx is:

Lx=Dz·Nx·FOV/Px=Dz·Nx·Cx,

-   -   where Cx is a constant for the camera arrangement in use.    -   (b) similarly, derive an approximation of the vertical lateral        displacement, Ly, in metres:

Ly=Dz·Ny·Cy,

-   -   again where Cy is a constant for that camera arrangement.    -   (c) derive a radial separation R between the detected object and        the HMD:

R=I√(Lx ² +Ly ² +Lz ²)

-   -   (d) compare R with a threshold value representing the detection        range.

A single lateral displacement Lr may be derived instead of separatehorizontal and vertical displacements, in dependence upon a number ofpixels along the shortest line, in the image, between the detectedobject and the HMD. Here, a replacement formula at step (c) could beR=√(Lr²+Lz²)

The comparison with a threshold distance may instead be carried outseparately in respect of lateral and depth displacements. Here, thelateral displacement Lr may be a radial lateral displacement in theimage either derived by combining Lx and Ly as √(Lx²+Ly²) or measureddirectly as just described. The comparison with a threshold could detectwhether either the lateral displacement Lr or the depth displacement Lzis less than a respective threshold, or could alternatively detectwhether both the lateral displacement Lr and the depth displacement Lzare less than respective thresholds. The respective thresholds could bethe same or different.

If an object is detected as being near to the HMD 700, this triggers thesteps of FIG. 16, with control passing to the step 630 at which theobject size or speed is compared with a threshold, and the remainder ofFIG. 16 is performed according to the results of that test.

Referring also to FIG. 15, the detection relating to the step 600 can becarried out by microphones 800 of the microphone array as shown in FIG.19.

FIGS. 23 to 25 schematic flowchart illustrating operations carried outin response to a detection. Here, the term “detection” relates to thesteps 930, 950 and 970 of FIGS. 23 to 25, and is generic to a detectionof an approaching object, a detection of a characteristic head movement,a detection of a characteristic sound or the like, all as discussed indetail above. In respect of FIG. 23, a response to such a detection mayinclude passing audio signals detected by the array of microphones 800to the earpieces of the HMD. In FIG. 24, a response to such a detectionmay include displaying a picture-in-picture (PIP) display so that animage of the surroundings (captured by a camera forming part of the HMD,the cameras 790 or both) is displayed as a sub-image or a partial imagewithin the overall display provided to the user by the HMD. In FIG. 25,a response to such a detection may include inhibiting a noisecancellation function relating to the earpieces associated with the HMD(noting that the earpieces may form part of the HMD or maybe part of aseparately worn headset).

Accordingly, embodiments of the invention can relate to a head mountabledisplay system comprising an HMD (such as the HMD 700 of FIG. 17 or anHMD as described earlier) and, separate to the HMD itself, an objectdetector for detecting the presence of an object within a detectionrange of the observer's head and a controller for controlling operationof the HMD so as to allow the observer to view at least a portion of thesurrounding real environment in response to detection of the presence ofan object within the detection range. The camera arrangement 720comprising the camera and microphone array 730 and the processor 740provide an example of the object detector and the controller, in thiscontext. Note however that even in the context of a separate camera, thefunction of the controller (and indeed of the object detector) could beimplemented at least in part by processing carried out at the HMD. Asignificant technical feature in these embodiments is that the camera isseparate to the HMD and is positioned so that, in use, it capturesimages of the HMD. In embodiments of the invention, the camera may be adepth camera (such as a stereoscopic camera as discussed above)configured to detect depth information associated with a captured image,though other alternatives have been described.

In embodiments of the invention the object detector is configured so asto detect the presence of an object within a detection range of theobserver's head. In the case of object detectors forming part of the HMDarrangement itself (such as a passive infra-red detector or the like),the detection range may be considered as relating to a range from theobject detector itself. In the case of embodiments by which objectdetection is carried out using a separate camera such as by using thecamera arrangement 720, the detection range may be considered as athreshold range detected by image and/or depth data analysis (laterally,in a depth direction, or in a radial direction relative to the detectedposition of the user's head derived from the lateral and depthdisplacements).

Other embodiments of an HMD system could incorporate the object detectorand controller functions (or at least parts of these functions) in theHMD itself, as described earlier.

It will be appreciated that various cameras and camera functions aredescribed in the present specification. The HMD itself may have an HMDcamera which (a) captures images for the surroundings of the HMD, forpossible display to the user, and/or (b) contributes to the detection ofHMD motion as discussed above. These functions may be carried out usingseparate HMD cameras or by the same HMD camera. More than one HMD cameramay be involved in either or both functions. The HMD may have nocameras. A different camera, such as that provided by the cameraarrangement 720, may be provided so as to provide images of the HMD inuse. This camera may be used for object detection as discussed above. Insome embodiments, images captured by the separate camera may be used asimages of the surroundings to be displayed to the user. More than oneseparate camera may be used. No separate camera could be used. In thebroadest aspects of the present embodiments, the question of whether acamera is part of the HMD or is a separate camera is not material. Inmore detailed expressions of the present embodiments, a camera may beconsidered as an HMD camera (forming part of and/or mounted on the HMD)or a non-HMD or separate camera.

Similarly, in respect of an audio in which transmit captured audiosignals to one or more transducers in response to a detection of one ormore characteristic sounds in a captured audio signal, the microphonearray (or other separate microphone or microphones not forming part ofthe HMD) could be used for either or both of (a) capturing an audiosignal in which characteristic sounds are detected, and (b) providing anaudio signal to be routed to the earpieces in case of such a detection.Accordingly, in examples of an audio system, all of the functionalitymay be provided at the headset, whereas in other examples, some of thefunctionality, including optionally some or all of the microphonefunctionality, may be provided at a separate unit or units. In such acase, the camera arrangement 720 may provide an example of a microphone,and some or all processing may optionally be carried out at theprocessor 740. Again, in the broadest aspects of the present technique,the choice of where the microphone or microphones is/are positioned, andthe choice of where the processing takes place, as between the headsetand a separate unit or units, are not material. In more closely definedarrangements, a microphone can be defined as a headset microphone or aseparate microphone. Note that the headset may be an HMD, oralternatively may not have any display capabilities.

A controller may be operable to transmit sounds from the user'senvironment to the user in response to a detection that one or more ofthe set of characteristic sounds is present in the captured audiosignal. This could be by transmitting the captured audio signal fromwhich the characteristic sounds were detected, to the user's earpieces.it could be by transmitting a separately captured audio signal to theuser's earpieces. So, for example, the detection and the user listeningcould be in respect of the same audio signal (captured by an HMDmicrophone or by a separate microphone). Alternatively, the detectioncould be in respect of an audio signal captured by one microphone (forexample, a separate microphone) but the transmission to the user'searpieces could be in respect of an audio signal captured by anothermicrophone (such as an HMD microphone). In further arrangements, thetransmission to the user could be implemented simply by temporarilyinhibiting the operation of a noise cancellation arrangement.

It will be appreciated that embodiments of the invention may beimplemented, at least in part, by software-controlled data processingapparatus. In such cases, the software, and a medium by which thesoftware is provided, for example a non-transitory machine-readablestorage medium such as a magnetic or optical disc medium or anon-volatile memory, are also envisaged as embodiments of the presentinvention.

It will be apparent that numerous modifications and variations of thepresent disclosure are possible in light of the above teachings. It istherefore to be understood that within the scope of the appended claims,the invention may be practised otherwise than as specifically describedherein.

1. An audio system comprising: a headset comprising a pair of earpiecespositionable with respect to a user's ears so that, in use, the user isinhibited from hearing sounds from the user's environment, at least oneof the earpieces having a transducer for reproducing sounds from aninput audio signal; a microphone for generating a captured audio signalrepresenting captured sounds from the user's environment; a detector fordetecting the presence of any of a set of one or more characteristicsounds in the captured audio signal; and a controller operable totransmit sounds from the user's environment to the user in response to adetection that one or more of the set of characteristic sounds ispresent in the captured audio signal.
 2. A system according to claim 1,in which both earpieces comprise a respective transducer, thetransducers being connectable to an audio signal source.
 3. A systemaccording to claim 1, in which the earpieces each comprise soundinsulating formations to inhibit transmission of sound from theenvironment to the user's ears.
 4. A system according to claim 1,comprising a noise reducing processor operable to generate an anti-phaseversion of an audio signal representing environmental noise, and totransmit the anti-phase version to the transducers for replay to theuser.
 5. A system according to claim 4, in which the microphone isoperable to capture the audio signal representing environmental noise.6. A system according to claim 1, in which the detector is operable todetect sounds of at least a threshold volume and/or having acharacteristic frequency content.
 7. A system according to claim 1, inwhich the microphone is separate to the headset and is positioned apartfrom the headset.
 8. A system according to claim 7, in which themicrophone comprises a phased microphone array.
 9. A system according toclaim 1, comprising a camera positioned apart from the headset so as tocapture images of the headset, the detector being operable to detect,from images captured by the camera, an object approaching the headsetwearer's head.
 10. A head-mountable display system comprising: a frameto be mounted onto a user's head, the frame defining one or two eyedisplay positions which, in use, are positioned for viewing by the user;a display element mounted with respect to each of the eye displaypositions, the display element providing a virtual image of a videodisplay of a video signal from a video signal source to the observer;and an audio system according to claim
 1. 11. A system according toclaim 10, in which the video signal source is a video gaming or dataprocessing machine.
 12. A system according to claim 10, in which, inuse, the virtual image is generated at a distance of more than one metrefrom the frame.
 13. A system according to claim 10, comprising: a secondcontroller, responsive to the detector, for controlling operation of thehead mountable display so as to allow the user to view at least aportion of the surrounding real environment in response to detection ofthe presence of an object within the detection range.
 14. A systemaccording to claim 13, in which the detector comprises: a camera mountedso as to move with the frame; and an image comparator operable tocompare successive images captured by the camera so as to detect thepresence, in a captured image, of an object that was not at that imageposition in a previously captured image.
 15. A system according to claim14, in which the detector comprises a passive infra-red object detector.16. A system according to claim 14, in which: the detector comprises anemitter and a receiver; the receiver is operable to detect reflectionsof signals emitted by the emitter; and the emitted signals compriseultrasonic, optical or radio frequency signals.
 17. A system accordingto claim 13, comprising a camera for capturing a view of the surroundingreal environment, and in which the second controller is operable tocause the display element to display at least a portion of the capturedview of the surrounding real environment in response to detection of thepresence of an object within the detection range.
 18. A system accordingto claim 13, in which the second controller is operable to control theone or more shutters to change between a view-obscuring mode and aview-allowing mode in which the one or more shutters allow the user toview at least a portion of the surrounding real environment.
 19. Asystem according to claim 13, comprising a motion detector for detectingmotion of the observer's head; in which the video signal source isoperable to provide a video signal for display having an apparentviewpoint dependent upon the detected motion.
 20. A method of operationof an audio system comprising a headset comprising a pair of earpiecespositionable with respect to a user's ears so that, in use, the user isinhibited from hearing sounds from the user's environment, at least oneof the earpieces having a transducer for reproducing sounds from aninput audio signal; the method comprising: generating a captured audiosignal representing captured sounds from the user's environment;detecting the presence of any of a set of one or more characteristicsounds in the captured audio signal; and transmitting the sounds fromthe user's environment to the user in response to a detection that oneor more of the set of characteristic sounds is present in the capturedaudio signal.
 21. A machine-readable, non-transitory storage mediumwhich stores computer software which, when executed by a computer,causes the computer to carry out the method of claim 20.